How to programmatically load a python script in Text Editor and launch it
Every Text Editor has its own active (visible) text datablock stored in .text.

To call the text.run_script() operator, use a context override and give it the right 'edit_text' member.

import bpy

text = bpy.data.texts.load(path_to_file)

for area in bpy.context.screen.areas:
    if area.type == 'TEXT_EDITOR':
        area.spaces[0].text = text # make loaded text file visible

        ctx = bpy.context.copy()
        ctx['edit_text'] = text # specify the text datablock to execute
        ctx['area'] = area # not actually needed...
        ctx['region'] = area.regions[-1] # ... just be nice

        bpy.ops.text.run_script(ctx)
        break